The instant invention is directed to a method for laser welding very thin foils or metals to achieve a vacuum tight seal. The method of the instant invention can also be used for welding foil seals for hydraulic and pneumatic systems. The method is specifically directed to the welding of metal foils which are less than 0.002 inches thick. Using the teachings of the instant invention the thin or ultra-thin metal foils are welded together using a laser welding and cutting operation. That is, the foils are welded while they are simultaneously trimmed, preferably by a pulsed laser, such as of the Neodymium:YAG type.
It is well recognized that the welding of metal foils having a thickness of less than 0.002 inches cannot be achieved using conventional laser welding methods. In fact, the prior art specifically teaches away from using a laser to achieve such thin metal welding. For example, U.S. Pat. No. 4,798,931, to Hess, III directed to "Simultaneously Cutting and Welding Sheet Metal Using Laser Energy", specifically states that the laser welding method of the patent "will not operate with extremely thin layers, and the patent is directed to metals with a range of thickness from 0.002 to 0.010 inches thick." Other prior patents recognize the difficulties encountered by welding even thicker metals. U.S. Pat. No. 4,546,230 to Sasaki displays graphically that the "welding of metal pieces is difficult unless the thickness thereof is 0.2 mm (0.008 inches) or above," (column 5). Similarly, U.S. Pat. No. 4,905,310 to Ulrich recognizes that a laser cannot be used to weld and cut very thin sheets and foils of metal because "such extremely thin layers may vaporize and/or curl and not function properly", (column 5).
Electrical resistance welding processes have also, in the past, been used for the welding of metal, but again such processes have proven suitable only for metal of a thickness of 0.003 inches or greater. Also, laser welding is preferred over electrical resistance welding as such a method would be more economical and more likely to provide a thicker, stronger, vacuum-tight, and error-free weld.
In the past, laser technology has been used with plastic and resin films. Such films have been simultaneously cut and welded using laser techniques. Although a laser has been successfully used with such materials, there is no suggestion in the prior art that the successful welding and cutting of very thin metal foils can be achieved by using similar methods. U.S. Pat. No. 4,945,203 to Soodak et al is an example of the use of a laser to simultaneously cut and weld film plastic bags. The reference uses a carbon-dioxide type laser to weld Teflon brand films having a thickness on the order of 0.001 inch to 0.10 inch. Even with the knowledge of the Soodak et al reference it is well known that the success of the method is limited to a plastic type of material and would not be successfully extended to very thin metal foils, as plastic materials have physical and chemical properties which are extremely different from those of metals. For example, plastics can be cut or welded in air at relatively low temperatures, whereas metals require much elevated temperatures. Metals further have a more complex polycrystalline structure and more complex processing parameters.
Similarly, U.S. Pat. No. 4,500,382 to Foster describes a method using a laser for simultaneously cutting resin film to the desired shape while simultaneously welding the edges. Again the prior art recognizes that such a method does not extend to very thin metal foils and teaches away from trying such a method with thin foil material.
Neodymium: YAG lasers and other solid state lasers, such as those suitable for use in the process of the instant invention, are well known and have been used for welding and cutting. U.S. Pat. No. 4,461,947 to Ward shows the use of such a laser for a similar purpose, but not for the simultaneous cutting and welding of very thin or ultra-thin foils. U.S. Pat. No. 4,564,736 to Jones discloses the use of a Neodymium: YAG laser in cutting, drilling, and welding processes, and U.S. Pat. No. 5,187,967 to Singh et al also discloses the use of a Neodymium: YAG laser for cutting. Again, none of the above cited patents utilize such a solid state laser for the simultaneous welding and cutting of very thin metal foils. None of the references recognize that such a procedure can be achieved through the use of a laser.
It is recognized that simultaneous cutting and welding can be achieved utilizing laser technology. As noted above both U.S. Pat. No. 4,500,382 to Foster, and U.S. Pat. No. 4,945,203 to Soodak disclose such a process with respect to resins or thin plastics although a vacuum tight weld or seal is not achieved. Likewise, simultaneous cutting and welding of metals is disclosed by U.S. Pat. No. 4,798,931 to Hess III, and U.S. Pat. No. 4,905,310 to Ulrich. The Hess III, and Ulrich patents, however teach that such methods do not work with very thin metals or foils.
Specifically, the Hess III and Ulrich patents disclose a method which "depends upon the creation of a globule or pool of molten metal, some of which is blown away and remainder of which is used to weld". The patents generally call for a space between the two layers to be welded. A narrow jet of air or inert gas is also required by the references to blow away some of the molten metal while some is left at the laser site. The spacing between the two metals to be welded is such that a globule of molten metal will bridge the gap and form a weld. In these two patents the focal point of the laser is set to be at the surface of the work, or below the surface. The patents suggest that for relatively thin materials the focal point be set at the surface of the work. The preferred embodiment of the patents is concerned with the welding of 321 stainless steel with a thickness of either 0.006 inches or 0.004 inches. No disclosure is made of an effective method for welding ultra-thin foils with a thickness of less than 0.002 inches. The references also do not disclose fixturing for directing heat away from the weld site to create pronounced weld beads.
As will be seen from the following description the process of the instant invention varies from that of the Hess III and Ulrich patents to satisfy a long felt need and achieve an improved result.